The present invention relates generally to transmission of data in a network. More particularly, the present invention relates to the transmission of signals over unshielded twisted pair transmission lines in a local area network in accordance with an improvement to IEEE Standard 802.3 (Ethernet).
Recent years have seen the rapid advance of networking of data processing systems in offices, educational institutions, and other environments. In response to a felt need for a widely adopted standard for interoffice communication, several industry groups and the IEEE adopted IEEE Standard 802.3 (Ethernet) for data transmission over unshielded twisted-pair at speeds up to 10 million bits per second (Mbps). The 10 Mbps transmission rate was accomplished by sending binary signals over wires at up to 20 Mhz, with the higher speed allowing for data buffering, error-checking and self clocking. The 802.3 standard has been widely adopted, resulting in a large installed base of hardware capable of transmitting 20 Mhz signals in office computer environments over unshielded twisted pair (UTP) wires. Part of this installed base of existing hardware is existing UTP wiring in many offices and educational institutions. Much of this UTP wiring is rated Category 3 by the Electronic Industries Association. Category 3 wiring is capable of carrying signals at frequencies up to approximately 30 Mhz. At higher frequencies, there is too much signal dispersion in Category 3 wiring and there is too much signal leakage from the wires. Among other problems, signal leakage results in RF interference that violates regulations of the Federal Communication Commission.
Ever increasing advances in computer technology as well as a desire to transmit graphics, video and other high bandwidth data over office computer networks have created a need for higher speed data transmission. Responding to this need, the IEEE in 1992 began to develop a standard for a local area network capable of transmitting data at 100 Mbps. According to this new proposed standard, data will be transferred from a media access controller to a physical layer transceiver as 4-bit nibbles at a clock speed of 25 Mhz and this data will be transmitted over the network at an effective data rate of 100 Mbps.
With the data rate of 100 Mbps set as a goal, the question arises of how to transmit the 100 Mbps of data to a remote receiver. The most straightforward method would be to increase the frequency on the channel carrying the data to something over 100 Mhz, thereby allowing for simple binary transmission of a 100 Mbps signal. However, as discussed above, the large installed base of UTP wires cannot handle signals at that high a frequency.
There are several known methods for transmitting a high binary data rate (like 100 Mbps) at a lower effective transmission frequency. One method reduces the frequency necessary to transmit a binary 100 Mbps signal by translating the binary signal to a signal with 5 or 6 discrete voltage levels before transmission. Signals with multiple voltage levels can inherently carry more data per clock period than a binary signal and so a lower frequency transmission signal may be used. Another known method divides up the data over multiple twisted pair cables, thereby reducing the signal on any one twisted pair while maintaining a higher overall data rate.
Both of these methods have a number of known disadvantages. A major source of these disadvantages is due to the fact that the clock speed at which data is transmitted between the network station and the network adaptor is different from the clock speed used by the adaptor to transmit data on the network. These two separate clock frequencies require two separate clock circuits and require that at least one of the circuits has a phase-lock-loop or other circuitry to keep the two clocks in alignment. Additionally, a network interface system using two different clock frequencies may need to provide some means of transferring data from one clock domain into the other. This transferal process takes additional time and may increase the access latency between the station and the network.
What is needed is a networking solution capable of transmitting binary data at a rate of 100 Mbps over existing office network transmission lines that avoids the problems inherent in interfaces using two different clock frequencies.